Physostigmine, also called eserine, and particular derivatives of physostigmine are anti-cholinesterase inhibitors which are well known. Such well known compounds are also useful in the treatment of glaucoma, Myasthenia, Gravis, Alzheimer's disease and as antidotes against poisoning with organophosphates.
Physostigmine was introduced into England in 1840 by Daniell (a British medical officer) in the form of the Calabar bean. The compound itself was first isolated by Jobst and Hesse in 1864. Physostigmine has been used as a treatment for glaucoma, and to reverse atropine-induced coma during the last century. Recent uses for this compound and its derivatives have been as effective antidotes to several drugs which possess central anti-cholinergic properties.
During the last two decades, studies related to the acetylcholine-receptor-ion-channel complex (AChR) of the neuromuscular junction have provided significant increases in knowledge of the receptor function. This membrane receptor has been readily available for study since nicotinic AChRs occur at very high densities in Torpedo and Electrophorus electric organs. Further, the understanding of the morphology and function of this receptor has been increased significantly by specific chemical probes for the different active sites of the receptor.
Nearly 20 years ago a significant discovery was made which helped in the study of this AChR. Alpha-bungarotoxin (Alpha-PGT) was obtained from snake venoms which binds irreversibly and specifically to the acetylcholine (ACh) recognition site on the nicotinic AChR. Alpha-PGT was such a highly selective probe that researchers were able to isolate and purify the different sub-units which comprise the nicotinic AChR. The sub-units were functionally reconstituted into artificial lipid membranes and were ultimately cloned.
Further sites on the nicotinic AChR were soon made available by the discovery of another class of toxins. These toxins were called histrionicotoxins and were isolated from the skin secretion of frogs in the family Dendrobatidae. The new sites available because of the hystrionicotoxins were discovered to be responsible for the allosteric alterations or non-competitive blockage of neuromuscular transmission. These sites are distinct from the against recognition site discovered through the alpha-PGT probe and are thought to be located on the ion channel component of the AChR.
Further, other drugs demonstrate the ability to modify non-competitively the activation of the AChR. Examples of such drugs are distinct and well known pharmacological agents which act on the peripheral nervous system as well as in the central nervous system. In particular, tricyclic anti-depressants, phenothiazine antipsychotics, the hallucinogenic agent Phencyclidine (PCP), local anesthetics, antimuscarinics, anticholinesterase agents and similar compounds to mention but a few.
Further ways for studying AChR are available due to microscopic kinetic models and biochemical rapid mixing methods to study permeability changes initiated by the binding of agonist molecules and conformational transitions of nicotinic receptor molecules.
The agonist recognition site at the nicotinic ACh receptor has been reported as having strong stereo specificity. This conclusion is based on the study of optical isomers of certain semi-rigid agonists, see for example Spivak et al., Mol. Pharmacol., Vol. 23, pages 337-343 (1983).
Conversely, the ion channel cites are apparently not stereo specific. This conclusion is based on the similar quantitative and qualitative actions of enantomers of perhydrohistrionicotoxin at the nicotinic AChR, see for example Spivak et al, FEBS Lett. Vol. 163, pages 189-193 (1983).
It has been discovered that the natural isomer of physostigmine has blocking properties as well as agonist properties at the neuromuscular AChR. By contrast (+)-physostigmine shows only negligible inhibition of cholinesterase (ChE). See Brossi et al., FEBS Lett., Vol. 201, pages 190-192 (1986).
Even though.(+)-physostigmine has only negligible ChE inhibitory activity it is every effective as a protective pretreatment drug against multiple lethal doses of sarin, see Albuquerque et al, Fundam. Appl. Caltoxicol., Vol. 5, pages 182-203 (1985). The observed beneficial protection appears to be due to direct interactions of the carbamates with the postsynaptic nicotinic AChR. The protective effectiveness of the carbamates against organophosphates appears to be related to the direct ability of the carbamates to decrease the hyperactivation caused by accumulation of the neurotransmitter.
The above information, available due to the research in this field, is important in the evaluation of potential new pharmacological agents for treating cholinergic disorders, for example, Myasthenia Gravis and Alzheimer's disease. Potential agents can be evaluated for potency in vitro by testing the agents against electric EEL acetylcholinesterase (AChE) and human plasma butyrylcholinesterase (BChE).
Of the two enzymes known to hydrolyze acetylcholine (ACh) in vivo, AChE, which is found in red blood cells, in the brain and in nerve tissues, seems to be more specific than BChE which is found in serum, pancreas and in the liver. It, however, has not previously been shown in the art that compounds which selectively inhibit one of the two enzymes more than the other would offer a medical advantage. The natural alkaloid (-)-physostigmine, its potential metabolite (-)-(Nl)-norphysostigmine, and the natural alkaloid physovenine which are used as biological standards in this art area inhibit AChE and BChE in vitro similarly at similar concentrations.
Accordingly, there is need in the art for highly selective agents active against one of AChE and BChE and not very potent against the other which may lead to better treatment of a particular cholinergic disorder and minimize negative side effects. Such compounds would be of great of medical importance in the treatment of cholinergic disorders.